The present invention relates to implantable medical devices (IMDs) that use a method for pacing a heart after an arrhythmia of a sustained duration to reduce the risk of stroke.
Arrhythmias are disorders of the regular beating of the heart. Irregular rhythms originating in one of the upper chambers of the heart, or atria, are called atrial arrhythmias, which include both atrial fibrillation and atrial flutter. One particular problem that can arise from a sustained period of atrial arrhythmia is the formation of blood clots, or thrombosis.
During atrial fibrillation, the heart's upper chambers quiver instead of beating effectively. Because blood is not pumped completely out of the atria during atrial fibrillation, blood may pool inside the atria, and a blood clot, or thrombus, may form on an atrial wall. When the normal heartbeat, or sinus rhythm, resumes, either spontaneously or through medical intervention, there is a danger that a blood clot that has formed in the left atrium will be dislodged. If a blood clot breaks free, it may become lodged in a blood vessel and cause a blockage, or thromboembolism, which may lead to stroke.
Since blood clots take time to form, there is a particular danger of thrombosis following arrhythmias of a sustained duration of time. Doctors seeking to treat patients by converting an irregular heart rhythm to a normal heart rhythm (cardioversion) must determine how long the arrhythmia has been taking place. If the patient's tachyarrhythmia has been for a sustained duration of time, usually defined as 48 hours or more, doctors may prescribe a blood thinning drug, such as heparin or warfarin, to dissolve any blood clots that may have formed. Cardioversion therapy is thus postponed until completion of the pharmacologic regimen.
A problem associated with cardioversion is early recurrence of atrial fibrillation (ERAF), which, as the name implies, means that the atrial fibrillation returns shortly after a sinus rhythm is achieved. Currently, implantable devices that provide high-energy shock therapy for termination of atrial fibrillation (such as the GEM III AT made by Medtronic, Inc.) and low-energy therapies for termination of atrial fibrillation (such as the GEM III AT and AT500 made by Medtronic, Inc.) are limited by ERAF. The risk of ERAF can be minimized by applying short-term, or transient, overdrive pacing. Overdrive pacing is a type of pacing therapy that paces the right atrium at a rate that is higher than the intrinsic heart rate.
Currently devices like the GEM III AT, AT500 and Kappa 900 (another device made by Medtronic, Inc.) include a feature called Post Mode Switch Overdrive Pacing (PMOP) that provides transient, high-rate overdrive pacing when a sinus rhythm is detected after an atrial arrhythmia. PMOP may decrease the risk of ERAF, but it is not intended to address two other problems that arise after cardioversion of an arrhythmia: stunned atrial myocardium and spontaneous echo contrast (SMOKE).
When a person suffers an arrhythmia for a sustained duration of time, the contractibility of the atria is depressed. This phenomenon is known as “stunned atrial myocardium”. During this period of time between the end of an arrhythmia and some later time when the patient has fully recovered, there is still a high risk of thrombosis. Spontaneous echo contrast (SMOKE) is a clinical parameter that is associated with this risk of thrombosis. It has been found that during this period of atrial myocardial stunning, applying high-rate atrial overdrive pacing resulted in a lower incidence of SMOKE, and therefore, presumably a lower risk of stroke. Sanders et al., “Reversal of Atrial Mechanical Dysfunction After Cardioversion of Atrial Fibrillation” (Circulation. 2003; 108:1976-1984).
Another type of atrial pacing is paired pacing (or coupled pacing), which is a pacing therapy that augments the contractility of the chambers of the heart. After termination of an arrhythmia, the heart requires a period of time until it fully recovers. Particularly, the cells of the heart exhibit a reduced capacity to contract, and as a result of this depressed contractility there is an increased risk of stroke. Paired pacing is the application of a pair of closely coupled pulses: an initial pulse and a second pulse just outside of the refractory period of the beat evoked by the initial pulse. Paired pacing increases the contractility of atrial tissue and reduces the time needed for atrial contractility to return to its normal state.
Hemodynamic sensor feedback has been used along with paired pacing, such as the device and method disclosed in U.S. Pat. No. 5,213,098. Also, a recent study demonstrated that atrial paired pacing may augment the function of the left ventricle function by improving atrial function and therefore augmenting ventricular filling. Gaasch et al., “Potentiation of Atrial Contractility by Paired Pacing Augments Ventricular Preload and Systolic Performance” (J Cardiac Fail. 2003; 8: 141-6).
Implantable devices that are capable of applying high rate overdrive pacing or paired pacing are known in the art. Using sensors to provide physiological feedback during pacing is also known. However, there is currently no implantable device that applies overdrive pacing or paired pacing in order to reduce the risk of stroke that occurs after a sustained period of atrial fibrillation. Also, there is also no implantable device that uses feedback from hemodynamic sensors to determine appropriate pacing parameters.